In a case of forming a superconducting band-pass filter having a microstrip line structure, it is preferable that resonators constituting a filter be a low loss and that a spurious frequency component, which is not intended in design, is suppressed. Particularly, in a case of forming a broadband band-pass filter, a strong coupling between the resonators constituting the filter is required.
An unloaded Q value Qu of the resonator is expressed as follows using a Q value Qc due to conductor loss, a Q value Qr due to radiation loss, and a Q value Qd due to dielectric loss:1/Qu=1/Qc+1/Qr+1/Qd 
In a case of forming the resonator of the microstrip line structure using conductor materials with low loss and a dielectric substrate with low loss, accordingly, a dominant factor that determines the unloaded Q value is the radiation loss. In order to realize the resonator with the low loss, therefore, it is important to suppress the radiation loss.